U.S. patent application number 12/716962 was filed with the patent office on 2010-06-24 for nitride semiconductor laser element.
This patent application is currently assigned to NICHIA CORPORATION. Invention is credited to Atsuo MICHIUE, Tomonori MORIZUMI, Hiroaki TAKAHASHI.
Application Number | 20100158066 12/716962 |
Document ID | / |
Family ID | 40221392 |
Filed Date | 2010-06-24 |
United States Patent
Application |
20100158066 |
Kind Code |
A1 |
MORIZUMI; Tomonori ; et
al. |
June 24, 2010 |
NITRIDE SEMICONDUCTOR LASER ELEMENT
Abstract
A nitride semiconductor laser element includes a nitride
semiconductor layer of a first conduction type, an active layer,
and a nitride semiconductor layer of a second conduction type that
is different from the first conduction type are laminated in that
order, a cavity end face formed by the nitride semiconductor
layers, and a protective film formed on the cavity end face. The
nitride semiconductor layers of the first and second conduction
types have layers containing Al, and the active layer has layer
containing In. The protective film has a region in which an axial
orientation of crystals is the same as that of the cavity end face
on the nitride semiconductor layers of the first and second
conduction types, and has another region in which an axial
orientation of crystals is different from that of the cavity end
face on the active layer.
Inventors: |
MORIZUMI; Tomonori;
(Anan-shi, JP) ; MICHIUE; Atsuo;
(Komatsushima-shi, JP) ; TAKAHASHI; Hiroaki;
(Anan-shi, JP) |
Correspondence
Address: |
GLOBAL IP COUNSELORS, LLP
1233 20TH STREET, NW, SUITE 700
WASHINGTON
DC
20036-2680
US
|
Assignee: |
NICHIA CORPORATION
Anan-shi
JP
|
Family ID: |
40221392 |
Appl. No.: |
12/716962 |
Filed: |
March 3, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12166746 |
Jul 2, 2008 |
7701995 |
|
|
12716962 |
|
|
|
|
Current U.S.
Class: |
372/49.01 |
Current CPC
Class: |
H01S 5/0281
20130101 |
Class at
Publication: |
372/49.01 |
International
Class: |
H01S 5/028 20060101
H01S005/028 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 6, 2007 |
JP |
2007-178915 |
Aug 29, 2007 |
JP |
2007-222613 |
Jun 20, 2008 |
JP |
2008-161731 |
Claims
1. A nitride semiconductor laser element comprising: nitride
semiconductor layers in which a nitride semiconductor layer of a
first conduction type, an active layer, and a nitride semiconductor
layer of a second conduction type that is different from the first
conduction type are laminated in that order; a cavity end face
formed by the nitride semiconductor layers; and a protective film
formed on the cavity end face, the nitride semiconductor layer of
the first conduction type and the nitride semiconductor layer of
the second conduction type having a layer containing Al, and the
active layer having a layer containing In, the protective film
having a region in which an axial orientation of crystals is the
same as that of the cavity end face on the nitride semiconductor
layer of the first conduction type and the nitride semiconductor
layer of the second conduction type, and having another region in
which an axial orientation of crystals is different from that of
the cavity end face on the active layer.
2. The element according to claim 1, wherein the protective film
has a region in which an axial orientation of crystals is different
from that of a contact interface of the nitride semiconductor
layers at the cavity end face with respect to the direction of
lamination of the protective film.
3. The element according to claim 1, wherein the protective film
has a region in which an axial orientation of crystals varies in
the direction of lamination of the protective film.
4. The element according to claim 1, wherein the protective film
has a hexagonal system crystal structure.
5. The element according to claim 1, wherein the protective film
has a region lattice-matched to the nitride semiconductor
layer.
6. The element according to claim 1, wherein the active layer is a
multiple quantum well structure comprising two or more well layers,
and the protective film is arranged such that the same axial
orientation of the crystals is continuous in the lamination
direction of the nitride semiconductor layer over the active layer
between the two well layers.
7. The element according to claim 1, further comprising an
additional protective film formed on the protective film, the
additional protective film being an amorphous film.
8. The element according to claim 1, wherein the active layer is
composed of InGaN, and the indium proportion is in a range of 0.01
to 0.30.
9. The element according to claim 1, wherein at least one of the
nitride semiconductor layer of the first conduction type and the
nitride semiconductor layer of the second conduction type is
composed of AlGaN.
10. The element according to claim 1, wherein the protective film
is composed of AlN.
11. A nitride semiconductor laser element comprising: nitride
semiconductor layers in which a nitride semiconductor layer of a
first conduction type, an active layer, and a nitride semiconductor
layer of a second conduction type that is different from the first
conduction type are laminated in that order; a cavity end face
formed by the nitride semiconductor layers; and a protective film
formed on the cavity end face, the nitride semiconductor layer of
the first conduction type and the nitride semiconductor layer of
the second conduction type having a layer containing Al, and the
active layer having a layer containing In, the protective film
having a region in which an axial orientation of crystals is
different from that of a contact interface of the nitride
semiconductor layers at the cavity end face with respect to the
direction of lamination of the protective film.
12. The element according to claim 11, wherein the protective film
has a region in which an axial orientation of crystals varies in
the direction of lamination of the nitride semiconductor
layers.
13. The element according to claim 11, wherein the protective film
has a region in which an axial orientation of crystals varies in
the direction of lamination of the protective film.
14. The element according to claim 11, wherein the protective film
has a hexagonal system crystal structure.
15. The element according to claim 11, wherein the protective film
has a region lattice-matched to the nitride semiconductor
layer.
16. The element according to claim 11, wherein the active layer is
a multiple quantum well structure comprising two or more well
layers, and the protective film is arranged such that the same
axial orientation of the crystals is continuous in the lamination
direction of the nitride semiconductor layer over the active layer
between the two well layers.
17. The element according to claim 11, further comprising an
additional protective film formed on the protective film, the
additional protective film being an amorphous film.
18. The element according to claim 11, wherein the active layer is
composed of InGaN, and the indium proportion is in a range of 0.01
to 0.30.
19. The element according to claim 11, wherein at least one of the
nitride semiconductor layer of the first conduction type and the
nitride semiconductor layer of the second conduction type is
composed of AlGaN.
20. The element according to claim 11, wherein the protective film
is composed of AlN.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of U.S. patent
application Ser. No. 12/166,746 filed on Jul. 2, 2008, now pending.
This application claims priority to Japanese Patent Application
Nos. 2007-178915, 2007-222613 and 2008-161731. The entire
disclosures of U.S. patent application Ser. No. 12/166,746, and
Japanese Patent Application Nos. 2007-178915, 2007-222613 and
2008-161731 are hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a nitride semiconductor
laser element, and more particularly relates to a nitride
semiconductor laser element having a protective film of a specific
structure on a cavity end face.
[0004] 2. Background Information
[0005] With a nitride semiconductor laser element, end faces of the
cavity formed by RIE (reactive ion etching) or cleavage has narrow
bandgap energy, so absorption of the exiting light occurs at the
end face, this absorption generates heat at the end face, and
problems such as a short service life are encountered in trying to
obtain a high-output laser. Consequently, there has been proposed,
for example, a method for manufacturing a high-output semiconductor
laser in which a silicon oxide or nitride film is formed as a
protective film on the cavity end face (see, for example, Japanese
Laid-Open Patent Application H10-70338). This protective film
functions as a window layer, and suppresses the absorption of light
at the cavity end face.
[0006] However, with a nitride semiconductor laser element, even if
the protective film is made from a material capable of suppressing
the absorption of light at the cavity end face, there will still be
a problem in that the desired function cannot be achieved because
the protective film separates or cracks develop in the laminated
nitride semiconductor layers due to a difference in the lattice
constants of the nitride semiconductors.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is simplified cross section illustrating the
structure of the laser elements of the present invention;
[0008] FIGS. 2A to 2C are simplified diagrams illustrating the
structure of the first protective film of the present
invention;
[0009] FIG. 3 is simplified a schematic TEM (transmission electron
microscope) image illustrating the cross sectional first protective
film of the present invention;
[0010] FIGS. 4A to 4J are schematic electron beam diffraction
images illustrating the state of axial orientation at the various
points on the first protective film shown in FIG. 3;
[0011] FIG. 5 is a graph showing the relationship of the
output-forward current before and after high-output drive of the
present invention;
[0012] FIG. 6 is a schematic TEM image illustrating the state of
axial orientation of another cross sectional first protective film
of the present invention;
[0013] FIG. 7 is a schematic TEM image illustrating the state of
axial orientation of still another cross sectional first protective
film of the present invention;
[0014] FIG. 8 is a schematic TEM image illustrating the state of
axial orientation of still another cross sectional first protective
film of the present invention;
[0015] FIG. 9 is a schematic image illustrating the state of axial
orientation of still another cross sectional first protective film
of the present invention.
SUMMARY
[0016] The present invention was conceived in light of this
problem, and it is an object thereof to provide a nitride
semiconductor laser element that has good characteristics, with
which the generation of cracks in the nitride semiconductor layer
is suppressed, and no separation of the protective film occurs at
the end face.
[0017] One aspect of the present invention provides a nitride
semiconductor laser element including nitride semiconductor layers,
a cavity end face and a protective film. The nitride semiconductor
layers has a nitride semiconductor layer of a first conduction
type, an active layer, and a nitride semiconductor layer of a
second conduction type that is different from the first conduction
type are laminated in that order. The cavity end face is formed by
the nitride semiconductor layers. The protective film is formed on
the cavity end face. The nitride semiconductor layer of the first
conduction type and the nitride semiconductor layer of the second
conduction type have a layer containing Al, and the active layer
has a layer containing In. The protective film has a region in
which an axial orientation of crystals is the same as that of the
cavity end face on the nitride semiconductor layer of the first
conduction type and the nitride semiconductor layer of the second
conduction type, and has another region in which an axial
orientation of crystals is different from that of the cavity end
face on the active layer.
[0018] Further, another aspect of the present invention provides a
nitride semiconductor laser element including nitride semiconductor
layers, a cavity end face and a protective film. The nitride
semiconductor layers has a nitride semiconductor layer of a first
conduction type, an active layer, and a nitride semiconductor layer
of a second conduction type that is different from the first
conduction type are laminated in that order. The cavity end face is
formed by the nitride semiconductor layers. The protective film is
formed on the cavity end face. The nitride semiconductor layer of
the first conduction type and the nitride semiconductor layer of
the second conduction type have a layer containing Al, and the
active layer has a layer containing In. The protective film has a
region in which an axial orientation of crystals is different from
that of a contact interface of the nitride semiconductor layers at
the cavity end face with respect to the direction of lamination of
the protective film.
[0019] With the above aspects of present invention, a protective
film formed on a cavity end face is disposed so as to have a region
of different crystal axial orientation in the lamination direction
of the nitride semiconductor layers, the result being that even
during high-output drive of the nitride semiconductor laser
element, load on the semiconductor layer due to stress of the
protective film is reduced, that is, fewer cracks develop in the
nitride semiconductor layer, and it is possible to prevent
separation of the protective film, etc. Accordingly, a high COD
(Catastrophic Optical Damage) level can be maintained without any
change occurring over time, and a nitride semiconductor laser
element with high performance, high output, and high reliability
can be provided.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0020] The inventors conducted diligent research into protective
films for cavity end faces in order to obtain good characteristics,
and particularly a higher COD level, even under high-output drive,
so that a nitride semiconductor laser element with higher
reliability could be obtained. As a result, they discovered that
the orientation of the protective film itself affects such things
as separation of the protective film and the load on the
semiconductor layer due to stress of the protective film, and more
particularly that there are changes over time in high-output drive.
In other words, although the COD level is high at the start of
drive when the orientation of the protective film is uniform, the
COD level decreases over time due to such factors as separation of
the protective film and load on the semiconductor layer as the
drive output rises.
[0021] Also, they discovered that, when there is a great difference
between lattice constant of the cavity end face which is an
exposure face of the stacked nitride semiconductor layers and
lattice constant of the protective film formed on the cavity end
face, the COD level decreases because the protective film formed on
the cavity end face is generated a light absorbing region composed
of polycrystalline and the like therein. This is affected by the
property of the protective film formed on a light waveguide region
which is formed within the nitride semiconductor layers, and
particularly an active layer.
[0022] This revealed that adjusting the orientation/lattice
constant of the protective film allows the characteristics of a
semiconductor laser element to be maintained over an extended
period, and this led to the perfection of the present
invention.
[0023] As typically shown in FIGS. 1 and 2A, for example, a nitride
semiconductor laser element of the present invention mainly
includes a semiconductor layers comprising a first nitride
semiconductor layer 12, an active layer 13, and a second nitride
semiconductor layer 14 in this order, and cavity end faces disposed
on opposed end faces of the semiconductor layers.
[0024] This nitride semiconductor laser element is usually formed
on a substrate 11, a ridge 16 is formed on the surface of the
second nitride semiconductor layer 14, and a first protective film
21 and a second protective film 22 are formed. Further, an embedded
film 15, a p-side electrode 17, an n-side electrode 20 are formed.
As shown in FIG. 1, an third protective film 18, a p-side pad
electrode, and so forth are formed.
[0025] As shown in FIGS. 2A, 2B, and 2C, the first protective film
is a film formed in contact with at least one cavity end face (and
particularly the end face where light exits, hereinafter sometimes
referred to as a "front side"). In this specification, hereinafter,
the protective film will also be referred to as a "first protective
film" and the first and second protective films which are formed on
the cavity end face will be referred to as an "end face protective
film." This first protective film has a region in which the axial
orientation of the crystals is different in the direction of
lamination of the nitride semiconductor layers. That is, the first
protective film does not have a uniform crystal axial orientation
in the in-plane, and the axial orientation instead varies.
Combinations of such difference axial orientations in the in-plane
encompasses various possibilities, such as a M axis (1-100)
orientation region and a C axis (0001) orientation region, or the M
axis orientation region and an A axis (11-20) orientation region,
or the M axis orientation region and the R axis (1-102) orientation
region, or the C axis orientation region and the A axis orientation
region, or the C axis orientation region and the R axis orientation
region, or the A axis orientation region and the R axis orientation
region, or the like.
[0026] Also, the first protective film may not have a uniform
crystal axial orientation in the direction of the thickness, and
the axial orientation instead varies. Combinations of the such
difference axial orientations in the direction of the thickness,
similar to the difference in the axial orientation in-plane,
encompasses various possibilities, such as a change from the M axis
(1-100) to the C axis (0001), or a change from the M axis to the A
axis (11-20), or a change from the M axis to the R axis (1-102), or
a change from the C axis to the A axis, or a change from the C axis
to the R axis, or a change from the A axis to the R axis, from the
cavity end face side to a surface side or in the reverse direction
thereof.
[0027] The phrase "axis orientation" here is not strictly limited
to a state in which a single crystal is oriented along the M axis,
C axis, A axis, or R axis, and may encompass a state in which
polycrystalline are also present, but a region (also, portion(s) or
part(s)) oriented along the M axis, C axis, A axis, or R axis are
included uniformly, or a state in which these are uniformly
dispersed. When there is thus a polycrystalline state, there will
not be a clear difference in the lattice constant from that of the
cavity end face, and this difference can be lessened.
[0028] This change in the axial orientation need not be a change to
a completely different axial orientation, and may instead be a
situation in which different axial orientations coexist, or in
which the proportion of a different axial orientation becomes
higher.
[0029] The axial orientation of the crystal in the first protective
film is preferably so as to have a region where the axial
orientation of the crystal is different from that of the cavity end
face at the contact interface with the cavity end face.
[0030] Because the crystal structure of the first protective film
of the present invention is such that there is a region where the
axial orientation is different within the in-plane and at least on
the contact side with the nitride semiconductor layers, stress is
divided at the interface between regions of differing axial
orientation, and stress is relieved within the first protective
film. Also, at the interface between regions of differing axial
orientation, there is a lattice constant differential and a
coefficient of thermal expansion differential. This, particularly
having the lattice constant differential at the interface between
regions of differing axial orientation, relieves stress in the
first protective film and thus to the nitride semiconductor layers,
prevents the first protective film from separating, and increases
the COD level.
[0031] The axial orientation of the first protective film can be
determined by the composition of the nitride semiconductor layers
of the cavity end face. For instance, as will be discussed below,
when the nitride semiconductor layer is formed from
In.sub.xAl.sub.yGa.sub.1-x-yN
(0.ltoreq.x.ltoreq.1,0.ltoreq.y.ltoreq.1), 0.ltoreq.x+y.ltoreq.1)
on the basis of GaN, as the composition of x increases, there is a
tendency for the axial orientation of the first protective film to
be different from that of the cavity end face. Also, as the
composition of y increases, there is a tendency for the axial
orientation of the first protective film to be same as that of the
cavity end face.
[0032] More specifically, when the cavity end face is the M plane,
that is, when the nitride semiconductor layers are oriented with C
axis orientation in the direction of lamination of the nitride
semiconductor layers, as shown in FIG. 9, M axial orientation and C
axial orientation are fairly well mixed in the region of the first
protective film 61 in contact with the GaN layer that is part of
the nitride semiconductor layers constituting the cavity end
face.
[0033] If indium proportion for only a tiny part of the InGaN
layer, then the properties of the GaN layer will be dominant in the
first protective film, and while M axial orientation and C axial
orientation will be fairly well mixed in the first protective film,
as the proportion of indium increases, the properties of the indium
will gradually come out in the first protective film, and the
indium properties will eventually become dominant, so that a region
appears in which substantially only C axial orientation is present
in the first protective film (See FIG. 9). The point at which the
indium properties appear here can be suitably adjusted by means of
the film formation method or the film thickness, for instance, but
an example of the indium proportion x is a range of about 0.01 to
0.30, and preferably about 0.01 to 0.20, and more preferably about
0.02 to 0.07.
[0034] In the case of an AlGaN layer, as the proportion of aluminum
increases, the properties of the aluminum gradually appear in the
first protective film, and eventually aluminum properties become
dominant, so that a region appears in which substantially only M
axial orientation is present in the first protective film (see FIG.
9). The point at which the aluminum properties appear here can be
suitably adjusted by means of the film formation method or the film
thickness, for instance, but an example of the aluminum proportion
y is a range of about 0.0001 or more, and preferably about 0.001 or
more, and still preferably about 0.01 or more.
[0035] The term "dominant" as used here indicates both that the
proportion is higher and that the properties are more pronounced
compared to those of others.
[0036] In FIG. 9, interfaces between a C axial orientation region
and a M axial orientation region, the C axial orientation and a
(M+C) axis orientation region are indicated by dotted line, but the
location of the dotted line can be shifted depend on the indium or
aluminum proportions in each nitride semiconductor layer. This
allows the first protective film in contact with the nitride
semiconductor layer which does not directly contribute to a light
emission to adhere well to the nitride semiconductor layer, whereas
it is possible to suppress the absorption of light at the first
protective film in contact with the nitride semiconductor layer
which does directly contribute to a light emission.
[0037] In particular, when the nitride semiconductor layer has an
In.sub.xGa.sub.1-xN (0<x.ltoreq.1) layer, the
In.sub.xGa.sub.1-xN (0<x.ltoreq.1) layer usually has a crystal
structure with C axial orientation in the lamination direction of
the nitride semiconductor layer, and the first protective film has
a crystal structure with C axial orientation in the direction
perpendicular to the cavity end face.
[0038] However, the axis orientation of the laminated nitride
semiconductor layers not necessarily may be partially or all of the
same as that of the first protective film at the contact interface
between them.
[0039] From another standpoint, when the nitride semiconductor
layer is formed from In.sub.xAl.sub.yGa.sub.1-x-yN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y .ltoreq.1, 0.ltoreq.x+y.ltoreq.1),
on the basis of GaN, a region in which substantially only C axial
orientation is present can be obtained in the first protective film
by setting the bandgap energy of the nitride semiconductor layers
to be the same or lower. Also, a region in which substantially only
M axial orientation is present can be obtained in the first
protective film by setting the bandgap energy of the nitride
semiconductor layers to be higher.
[0040] From yet another standpoint, when the nitride semiconductor
layer is formed from In.sub.xAl.sub.yGa.sub.1-x-yN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.x+y.ltoreq.1),
on the basis of GaN, a region in which substantially only C axial
orientation is present can be obtained in the first protective film
by setting the lattice constant of the nitride semiconductor layer
to be the same or larger. Also, a region in which substantially
only M axial orientation is present can be obtained in the first
protective film by setting the lattice constant of the nitride
semiconductor layer to be smaller, on the basis of GaN.
[0041] That is, if the difference between the lattice constant of
the well layers that make up the active layer and the lattice
constant of the first protective film is to be reduced, then the
axial orientation of the first protective film may be different
from the axial orientation of the well layer exposed at the cavity
end face, in other words, these may not be lattice-matched. If the
difference between the lattice constant of the nitride
semiconductor layer exposed as the cavity end face and the lattice
constant of the first protective film formed at the cavity end face
is reduced, it is believed that optical absorption can be
suppressed at the contact interface between these, and that the COD
level can thus be increased.
[0042] Also, the first protective film may be lattice-matched to
the nitride semiconductor layer. However, there is no need for
lattice matching over the entire contact interface between the
nitride semiconductor layer and the first protective film.
[0043] This differences in axial orientation within the in-plane of
the first protective film is, in one embodiment, believed to appear
or vary due to the axis orientation of the nitride semiconductor
itself contact with the first protective film or composition as
well as a difference in bandgap, or in lattice constant, or in the
thickness or in the composition of the nitride semiconductor layer
of the adjacent the first protective film. Therefore, these
variation and differences in axial orientation of the first
protective film tend to occur at the lamination interface of the
nitride semiconductor layer and in the vicinity thereof.
[0044] For example, when the nitride semiconductor layer is an
InGaN layer, and an AlGaN layer is disposed on one side thereof,
the first protective film in contact with the InGaN layer tends to
be affected by the adjacent AlGaN layer, and it has been confirmed
that as the aluminum content rises, the effect of the AlGaN layer
increases.
[0045] Therefore, the change in axial orientation and the
differences of the axial orientation in the in-plane of the first
protective film need not necessarily occur at all the lamination
interfaces of the nitride semiconductor layers.
[0046] More specifically, the differences of the axial orientation
in the in-plane of the first protective film tends to form at the
interface with the active layer and the nitride semiconductor layer
of the first conduction type, and at the interface with the active
layer and the nitride semiconductor layer of the second conduction
type. Also, if the active layer has a quantum well structure, this
difference tends to form at one or more interfaces with barrier
layer and the well layer, and at the interface with the outermost
barrier layer and the well layer and in the vicinity thereof
easily. Stress can be relieved in the first protective film by
having the axial orientation in the in-plane of the first
protective film be different at the active layer or on the optical
waveguide including the active layer, from that at the nitride
semiconductor layer laminated above and below the active layer, on
the cavity end face formed by the nitride semiconductor layers.
[0047] Over the cavity end face formed in the nitride semiconductor
layers, the axial orientation in the in-plane of the first
protective film is preferably different from that of the cavity end
face at the active layer or on the optical waveguide including the
active layer, and preferably at the well layer and in the vicinity
of the barrier layer laminated above and below the well layer, and
is preferably the same as that of the cavity end face at the
nitride semiconductor layer laminated above and below the active
layer. Having the axial orientation be the same as that of the
cavity end face over the nitride semiconductor layer laminated
above and below results in good adhesion between the first
protective film and the nitride semiconductor layer. Also, if the
first protective film is formed with different axial orientation
from that of the cavity end face at the active layer or on the
optical waveguide including the active layer, then a region will be
formed with different axial orientation in the in-plane direction
of the first protective film, which cause an interface between
different axial orientation regions, so stress is relieved in the
first protective film. As a result, stress attributable to heat
caused by irradiation with laser light at the cavity end face,
particularly near the active layer, can be relieved by difference
of the axial orientation of the first protective film, and this
also effectively prevents separation of the first protective film,
loading of the nitride semiconductor layer, etc.
[0048] When a plurality of well layers is used, the first
protective film is preferably formed in a different axial
orientation from that of the cavity end face, continuously between
the well layers at both ends.
[0049] In particular, when the active layer has a multiple quantum
well structure comprising two or more well layers, it is preferable
if the first protective film has the same axial orientation of the
crystals, continuously in the lamination direction of the nitride
semiconductor layers over the active layer between the well layers
located on both sides (see FIG. 8).
[0050] The first protective film may have a region in which the
crystal axial orientation is different from that of the cavity end
face in the thickness direction of the first protective film from
the contact interface with the well layer (see FIGS. 3 and 6,
etc.). This allows stress to be relieved in the thickness direction
of the first protective film as well.
[0051] The first protective film may be a film composed of oxides
(Si, Mg, Al, Hf, Nb, Zr, Sc, Ta, Ga, Zn, Y, B, Ti, etc.), nitrides
(AlN, AlGaN, GaN, BN, SiN, etc.) or fluorides and the like. Among
these, the first protective film preferably includes the crystal
structure of hexagonal system, and composed of AlN.
[0052] The protective film may be formed, for example, about 5 to
about 500 nm, and preferable about 5 to about 100 nm. Also, the
protective film may be formed not only on the end face where light
exits (front side) of the cavity, but also on the end face where
light reflects (hereinafter sometimes referred to as a "rear side"
which is the face on the opposite side).
[0053] The state of the first protective film is generally
classified as monocrystalline, polycrystalline, or amorphous,
depending on the degree of crystallization of the material that
makes up the film. Monocrystallines have almost no variation in
lattice constant among materials, and there is almost no lattice
plane inclination. To put this another way, the atoms in the
material are arranged in a regular pattern, and order is maintained
over an extended distance. Polycrystallines are made up of numerous
microscopic monocrystallines, i.e., microcrystallines. An amorphous
material is one that has no periodic structure such as that in a
crystal, that is, it means that the atomic arrangement is irregular
and there is no order over an extended distance.
[0054] The state of the film (crystalline or a crystal state in the
case of a crystal substance) can be easily evaluated from a
diffraction image produced by electron beam. How the elements that
make up the crystals are arranged can be grasped visually from the
resulting electron beam diffraction image.
[0055] That is, an electron beam is directed at the film, and an
electron beam diffraction image appears corresponding to the planar
direction and the size of the lattice constant. For instance, in
the case of a monocrystalline, since the crystal planes are almost
aligned, the diffraction points are observed to align with good
regularity. In the case of a polycrystalline, since it is made up
of microcrystals, the lattice planes are not all facing the same
way, and the diffraction points may come together in a complex
fashion, or Debye rings may be seen. In the case of an amorphous
material, meanwhile, since the atomic arrangement does not have a
periodic structure over an extended distance, no electron beam
diffraction occurs. Therefore, this is observed as a state in which
the diffraction image has no diffraction points.
[0056] That crystallinity is different, and that the axial
orientation of the crystal structure is different can be confirmed
not only by TEM, STEM (Scanning Transmission Electron Microscope),
SEM (Scanning Electron Microscope), and other kinds of cross
section observation (bright field, selected-field, high resolution
and the like), but also by electron beam diffraction or the product
of subjecting these patterns to FFT (Fast Fourrier Transform), or
from the difference in the etching rate.
[0057] In other words, in observing a protective film under a
microscope, a difference can be visually ascertained, which is due
to a difference in crystallinity, between a region adjacent the
active layer and a region in contact with the first and second
nitride semiconductor layers.
[0058] In particular, in observation by STEM, TEM or the like, a
contrast (bright or dark) is observed due to the different states
of the film (crystalline or a crystal state in the case of a
crystal substance).
[0059] Also, even when the same film is observed, contrast will
sometimes be observed to be inverted when the observation
conditions (STEM or TEM image display settings) are changed.
[0060] An electron beam diffraction image can be observed by
cutting the protective film so that a cross section is exposed with
respect to the end face where the protective film is formed, and
directing an electron beam at this cross section. The electron beam
diffraction image observation can be carried out, for example,
using a JEM-2010F made by JEOL.
[0061] Observation is conducted by the following procedure. First,
a specimen is cut out by microprobing using a focused ion beam
(FIB) machining apparatus (for example, SMI3050MS2 made by Seiko
Instruments Inc.), and a thin film (for example, at least about 100
nm) is obtained by subjecting the specimen to FIB machining.
Further thin film (for example, at least about 50 nm) working is
performed by ion milling. Then, a dark field image can be obtained
by performing TEM observation at a prescribed acceleration voltage
(for example, about 200 kV).
[0062] Furthermore, if the first protective film thus obtained is
immersed in a suitable etchant, such as an acid solution (for
example, buffered hydrofluoric acid or the like) or alkali solution
(for example, KOH or the like), a difference in crystallinity can
be discerned from a difference in solubility (etching rate
difference). In this etching, a material with poor crystallinity
will be quickly dissolved or removed, while a material with good
crystallinity will remain or be preserved.
[0063] These are not the only methods that can be used, and the
crystallinity of the first protective film can be evaluated using
any known method.
[0064] The first protective film can be formed, for example, by a
method that is known in this field. For instance, this can be vapor
deposition, sputtering, reactive sputtering, ECR (electron
cyclotron resonance) plasma sputtering, magnetron sputtering, ion
beam assist deposition, ion plating, laser ablation, CVD (Chemical
Vapor Deposition), spraying, spin coating, dipping, a combination
of these two or more methods, a combination of these methods and
oxidation treatment (thermal treatment), or any of various other
methods, and preferably ECR plasma sputtering.
[0065] Although it will depend on the film formation method, it is
preferable to control film formation by subjecting the end face of
the cavity to a nitrogen plasma treatment prior to film formation,
or to adjust the film formation rate to a relatively first rate, or
to control the atmosphere during film formation (to reduce a
nitrogen gas partial pressure to the extent that the protective
film does not have absorption, for example), or to adjust the film
formation pressure to a relatively high level, etc. Two or more of
these methods may also be combined. Also, the nitrogen partial
pressure may be gradually or abruptly changed during the film
formation in each method, or the film formation pressure may be
gradually or abruptly changed.
[0066] Examples include a method in which, in forming a film by
sputtering using the first protective film material as a target,
the film formation rate is gradually or suddenly increased, or the
RF power is gradually or suddenly increased (with the range of
increase being about 100 to 1000 W), or the distance between the
target and the substrate is gradually or suddenly changed (with the
range of change being about 0.2 to 3 times the original distance),
and a method in which the pressure is gradually or suddenly
decreased (with the range of decrease being about 0.1 to 2.0 Pa) in
forming a film using the first protective film material as a
target.
[0067] For example, to obtain a film by using ECR plasma sputtering
method, it is preferably to adjust a film at a film formation rate
of 0.5 to 10 nm/min. The microwave power is preferably 300 to 1000
W, the RF power is 100 to 1000 W.
[0068] Also, a method in which the temperature of the substrate is
gradually or suddenly increased or lowered (with the range of
change being about 50 to 500.degree. C.) may be used. Then, thermal
treatment may be performed.
[0069] When an AlN film is formed by ECR sputtering, if the cavity
end face is the M plane, orientation of the film can be along the M
axis (coaxial) and the C axis (stable). The film formation rate has
to be controlled in order to control the orientation of the film,
and M axial orientation is more easily continued if the film
formation rate is lowered. Examples of ways to lower the film
formation rate include lowering the RF power, lowering the film
formation gas pressure, and raising the nitrogen gas partial
pressure. It is also possible to form AlN with partial M axial
orientation and C axial orientation by raising the film formation
rate by raising the RF power, lowering the film formation gas
pressure, lowering the nitrogen gas partial pressure, etc. It is
preferable for an AlN film to be formed such that, depending on
these conditions, the axial orientation of crystals in the
lamination direction of the nitride semiconductor layers is
different in M axial orientation and C axial orientation in the
first protective film formed on the cavity end face. This relieves
stress to the nitride semiconductor, prevents the first protective
film from separating, and allows the COD level to be kept high even
after high-output drive.
[0070] Furthermore, these methods can be combined as desired.
[0071] Examples of preferable conditions for forming the first
protective film include a film formation rate of about 2.5 to 10
nm/min, a microwave power of 400 to 1000 W, and an RF power of
about 400 to 1000 W. The use of argon, krypton, xenon, or another
such rare gas is preferable for the atmosphere gas. When a film of
AlN is to be formed, it is preferable to use an aluminum target and
set the flow of nitrogen (nitrogen raw material) to between about 3
and 8 sccm, and the flow of atmosphere gas to between about 25 and
50 sccm. Furthermore, differences in axial orientation and/or
lattice constant in the in-plane and/or thickness direction of the
first protective film can be achieved by combining and suitably
adjusting these parameters.
[0072] In the nitride semiconductor laser of the present invention,
it is preferably formed a second protective film 22 on the first
protective film 21 as the end face protective film, as shown in
FIG. 2A. The second protective film can make the first protective
film to be more forcibly adhered into the cavity end face.
[0073] The second protective film may be a film composed of oxides
of Si, Mg, Al, Hf, Nb, Zr, Sc, Ta, Ga, Zn, Y, B, Ti, etc.,
preferably be a film composed of SiO.sub.2.
[0074] The second protective film may be has any of single layer or
laminated structure. The second protective film is preferably an
amorphous film. This divides the stress at the interface where the
axial orientation of the crystals formed by the first protective
film is different, allows stress to be released better, and
improves the adhesion of the first protective film.
[0075] The thickness of the second protective film is preferably
greater than the thickness of the above-mentioned first protective
film. For example, the combined thickness of the above-mentioned
first protective film and the second protective film may be 0.1 to
2 .mu.m or less. The result is that the above-mentioned effects are
more pronounced.
[0076] As shown in FIGS. 2B and 2C, the first protective film
and/or the second protective film may be made of different
materials, have different thicknesses, etc., on the exit side and
the reflecting side. The second protective film on the exit side is
preferably formed from a single layer of SiO.sub.2. Examples of the
second protective film on the reflecting side include a laminated
structure of SiO.sub.2 and ZrO.sub.2, and a laminated structure of
Al.sub.2O.sub.3 and ZrO.sub.2. The lamination period and so forth
can be adjusted as needed, according to the desired
reflectivity.
[0077] The amorphous second protective film is similar to the
above-mentioned first protective film in that it can be formed by
any of the known methods listed above, etc. In particular, to
create an amorphous film, although it will depend on the film
formation method, it is preferably to control film formation by
adjusting the film formation rate to be faster, or controlling the
atmosphere during film formation to be an oxygen atmosphere, or
adjusting the film formation pressure higher, or the like, or by
combining two or more of these methods. When an oxygen atmosphere
is used, the oxygen is preferably introduced to the extent that
absorption is not induced. Specific film formation conditions are
preferably such that the film is formed using a silicon target in
an ECR plasma sputtering device, at an oxygen flow of 3 to 20 sccm,
a microwave power of 300 to 1000 W, and an RF power of about 300 to
1000 W.
[0078] In the present invention, a substrate for forming the
nitride semiconductor laser may be an insulating substrate or a
conductive substrate. The substrate is, for example, preferably a
nitride semiconductor substrate having an off angle of no more than
10.degree. and greater than 0.degree. to the first main face and/or
the second main face. The thickness of the substrate is at least 50
.mu.m and no more than 10 mm, for example. A commercially available
substrate, any of the various known substrates disclosed, for
instance, in Japanese Laid-Open Patent Application 2006-24703, or
the like may be used.
[0079] The nitride semiconductor substrate can be formed by a vapor
phase growth method such as MOCVD (Metal Organic Chemical Vapor
Deposition), HVPE (Hydride Vapor Phase Epitaxy), MBE (Molecular
Beam Epitaxy), or the like, a hydrothermal synthesis method in
which crystals are grown in a supercritical fluid, a high pressure
method, a flux method, a melt method, or the like.
[0080] The nitride semiconductor layer may include a layer having a
general formula of In.sub.xAl.sub.yGa.sub.1-x-yN
(0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1, 0.ltoreq.x+y.ltoreq.1).
In addition to this, it may be used the semiconductor layer which
is partly substituted with B as a group III element, or is
substituted a part of N as a group V element with P or As.
[0081] The n-side nitride semiconductor layer may doped with at
least one n-type impurity of IV or VI group elements, such as Si,
Ge, Sn, S, O, Ti, Zr, Cd etc. The p-side nitride semiconductor
layer may doped with at least one p-type impurity, such as Mg, Zn,
Be, Mn, Ca, Sr etc. The doped concentration is, for example, about
5.times.10.sup.16/cm.sup.3 to about 1.times.10.sup.21/cm.sup.3. As
for the first conductivity and the second conductivity, which may
be n-type or p-type. All of layers in the n-type or p-type nitride
semiconductor layers may not necessarily contain n-type or p-type
impurity.
[0082] The nitride semiconductor layers of the first and second
conductivity type can be formed as a layer having a function of
clad, guide, cap, contact, clack preventing, or the like with a
single layer, multilayer, or super lattice structure in position in
conjunction with an appropriate structure or composition in order
to obtain a semiconductor laser element having a desired
characteristics.
[0083] Also, at least one of the nitride semiconductor layers of
the first and second conductivity type that make up the structure
may exhibit n-type or p-type conductivity, all of the layers need
to exhibit n-type or p-type conductivity.
[0084] The active layer may be a multiple quantum well or single
quantum well structure. The active layer preferably comprises a
layer which includes indium, and it is suitable that the
composition is an average mixed crystal with an indium component of
more than 0.5 and 15% or less, and preferably from 0.5 to 10%, and
more preferably 0.5 to 7%.
[0085] The nitride semiconductor layer preferably has a structure
which is a SCH (Separate Confinement Heterostructure) wherein an
optical waveguide is constituted by providing n-side and p-side
optical guide layers above and below the active layer. However,
there is no particular restriction on these structures.
[0086] In the nitride semiconductor laser element of the present
invention may emits laser light with a wavelength of about 370 to
500 nm, it is possible to prevent separation of the end face
protective films and to improve COD level.
[0087] There is no particular restriction on a growth method of the
nitride semiconductor layer, it can be formed by means of any known
method which can grow these nitride semiconductor layers, such as
MOVPE (Metal Organic Vapor Phase Epitaxy), MOCVD, HVPE, MBE or the
like. In particular, MOCVD is preferable because it allows the
nitride semiconductor to be growth with good crystallinity.
[0088] A ridge is formed on the surface of the second conductivity
type nitride semiconductor layer. The ridge functions as an optical
waveguide, the width of the ridge may be from about 1.0 to 30.0
.mu.m, about 1.0 to 8.0 .mu.m, and preferably about 1.0 to 3.0
.mu.n. The height of the ridge (the etching depth) may be, for
example, may be from about 0.1 to 2 .mu.m. The extent of optical
confinement can be suitably adjusted by adjusting the thickness,
material, and so on of the layer that makes up the second
conductivity type semiconductor layer. The ridge is preferably set
so as to be about 200 to 5000 .mu.m of cavity length. The ridge
need not be all the same width in the extension direction of the
cavity, and its side faces may be vertical or may be tapered with
an angle of about 45 to 90.degree..
[0089] The cavity end face formed by the nitride semiconductor
layers may be either the M plane (1-100), A plane (11-20), C plane
(0001), or R plane (1-102), but the M plane is preferred. This is
because the face can be formed easily and precisely by
cleavage.
[0090] An embedded film is usually formed on the surface of the
nitride semiconductor layer and to the side faces of the ridge.
That is, the embedded film is above the nitride semiconductor
layer, and is formed in a region other than the region where an
electrical connection between the nitride semiconductor layer and
an electrode (discussed below) is made. There are no particular
restrictions on the position, size, shape, etc., of the region of
connection between the nitride semiconductor layer and the
electrode, but this region may, for example, be part of the surface
of the nitride semiconductor layer, such as substantially the
entire top face of the stripe-like ridge formed on the surface of
the nitride semiconductor layer.
[0091] The embedded film is generally formed from an insulating
material with a smaller refractive index than that of the nitride
semiconductor layer. The refractive index can be measured using a
polarizing ellipsometer (featuring ellipsometry), more
specifically, it is, for example, HS-190 made by J. A. WOOLLAM and
other ellipsometers. This embedded film is an insulator of a
dielectric film of single layer or multilayer film composed of
oxides, nitrides or oxide-nitrides of Zr, Si, V, Nb, Hf, Ta, Al,
Ce, In, Sb, Zn and the like. The embedded film may have
monocrystalline, polycrystalline or amorphous structure. If the
embedded film is formed from the side faces of the ridge all the
way to the surface of the nitride semiconductor on both sides of
the ridge, it will ensure a refractive index difference versus the
nitride semiconductor layer, and particularly the second
conductivity type semiconductor layer, which allows leakage of
light from the active layer to be controlled, allows light to be
confined efficiently within the ridge, and also better ensures
insulation near the base of the ridge, so the generation of leak
current can be avoided.)
[0092] This embedded film can be formed by any method that is known
in this field. For instance, a variety of methods can be used, such
as vapor deposition, sputtering, reactive sputtering, ECR plasma
sputtering, magnetron sputtering, ion beam assist deposition, ion
plating, laser ablation, CVD, spraying, spin coating, dipping, a
combination of these two or more methods, a combination of these
methods and oxidation treatment (thermal treatment), or the
like.
[0093] Electrodes of the nitride semiconductor laser element of the
present invention are a pair of the electrode which is electrically
connected with the first and second conductivity type nitride
semiconductor layers, respectively.
[0094] The p-side and an n-side electrodes may preferably be formed
with a single layer or laminated layer of a metal or metal alloy of
palladium, platinum, nickel, gold, titanium, tungsten, cupper,
silver, zinc, tin, indium, aluminum, iridium, rhodium, ITO (Indium
Tin Oxide) or the like. The electrodes are suitable formed in a
thickness of for example, about 50 to about 500 nm.
[0095] The electrode connected with the second conductivity type
nitride semiconductor layer is preferably formed over the nitride
semiconductor layer and the embedded film.
[0096] The electrode connected with the first conductivity type
nitride semiconductor layer may be formed either directly on the
first conductivity type nitride semiconductor layer, i.e., formed
on the same side with a p-side electrode with respect to the
substrate, or on the substrate.
[0097] A third protective film is preferably formed on the embedded
film. This third protective film may be disposed over the embedded
film on at least the surface of the nitride semiconductor layer,
and preferably also covers the side faces of the nitride
semiconductor layer and/or the side faces, surface, etc., of the
substrate with or without the embedded film interposed
therebetween. The third protective film can be formed from the same
materials as those listed as examples for the embedded film. As a
result, it is possible to ensure not only insulation reliably but
also protection for the exposed side faces, surface, etc., of
nitride semiconductor layer.
[0098] A conductive layer such as a pad electrode of single layer
or laminated layer may be formed on the embedded film, electrode
and the third protective film over the side faces of the nitride
semiconductor layers up to the top face.
[0099] Examples of the nitride semiconductor laser element of the
present invention will now be described in detail through reference
to the drawings. The present invention is not, however, limited to
or by these examples.
EXAMPLE 1
[0100] As shown in FIGS. 1 and 2C, the nitride semiconductor laser
element of this Example comprises the first nitride semiconductor
layer 12, the active layer 13, and the second nitride semiconductor
layer 14 on the surface of which is formed the ridge 16, laminated
in that order on the substrate 11, and a cavity is formed therein.
With this nitride semiconductor laser element, a first protective
film 21 and a second protective film 22 are formed on a light exit
side of a cavity end face, and a first protective film 21a and a
second protective film 22a are formed on a light reflecting side of
the cavity are formed, and also an embedded film 15, a p-side
electrode 17, an n-side electrode 20, a third protective film 18, a
pad electrode 19 and the like are formed.
[0101] The cavity end faces are formed by a nitride semiconductor
layers having M axial orientation, and the first protective film is
composed of AlN in which the axial orientation is different in the
in-plane, i.e., the axial orientation changes in the in-plane, and
has a thickness of about 30 nm.
[0102] This laser element can be manufactured by the following
method (See, FIG. 3).
[0103] First, a GaN substrate (not shown) is provided. In a
reaction vessel, an n-side clad layer 12b composed of
Al.sub.0.03Ga.sub.0.97N doped with Si at 4.times.10.sup.18/cm.sup.3
(2 .mu.m thick) is grown on the GaN substrate at a growth
temperature of 1160.degree. C. using trimethylaluminum (TMA),
trimethylgallium (TMG) and ammonia (NH.sub.3) as the raw material
gas with a silane gas for an impurity gas. This n-side clad layer
12b may be composed of a super lattice structure.
[0104] Next, the silane gas and TMA is stopped, and n-side wave
guide layer 12a composed of undoped GaN (0.175 .mu.m thick) is
grown at a growth temperature of 1000.degree. C. This wave guide
layer 12a may be doped with n-type impurities.
[0105] The temperature is set to 900.degree. C., trimethylindium
(TMI) is used, a barrier layer 13b composed of
In.sub.0.02Ga.sub.0.98N doped with Si (14 nm thick) and at same
temperature, a well layer 13a composed of undoped
In.sub.0.07Ga.sub.0.93N (8 nm thick) are laminated on the barrier
layer 13b. This process is repeated 2 times, finally an undoped
barrier layer 13b is formed on the layers to grow an active layer
13 composed of a multiple quantum well structure (MQW)) with a
total thickness of 58 nm.
[0106] TMI is stopped, a p-side cap layer 14a composed of
p-Al.sub.0.2Ga.sub.0.8N doped with Mg at 1.times.10.sup.20/cm.sup.3
(10 nm thick) is grown on the active layer 13 using TMG, TMA,
NH.sub.3, and Cp.sub.2Mg (bis-cyclopentadienyl magnesium) at a
growth temperature of 1000.degree. C. This p-side cap layer 14a can
be omitted.
[0107] Next, Cp.sub.2Mg gas and TMA are stopped, and p-side wave
guide layer 14b composed of undoped GaN (0.145 .mu.m thick) is
grown at a growth temperature of 1000.degree. C.
[0108] The temperature is set to 1000.degree. C., and an A layer
composed of undoped Al.sub.0.10Ga.sub.0.90N (2.5 nm thick) is
grown, and then TMA is stopped and Cp.sub.2Mg gas is used, a B
layer composed of GaN (2.5 nm thick) is laminated. The A layer and
the B layer are alternately laminated, and this process is repeated
to grow a p-side clad layer 14c composed of a super lattice
structure with a total thickness of 0.4 .mu.m.
[0109] Finally, a p-side contact layer (not shown) composed of GaN
doped with Mg at 1.times.10.sup.20/cm.sup.3 (15 nm thick) is grown
on the p-side clad layer 14c at a growth temperature of
1000.degree. C.
[0110] The resulting wafer on which the nitride semiconductor has
been grown is taken out of the reaction vessel, and a mask composed
of SiO.sub.2 is formed on the surface of the p-side contact layer
(the outermost layer). And the nitride semiconductor layers are
etched using the mask to form a stripe-shaped structure of the
nitride semiconductor with a length, which corresponds to cavity
length, of 800 .mu.m. This portion will be main body of the cavity
in the laser element. The cavity length may be preferably set to
the range from 200 to 5000 .mu.m.
[0111] Next, a stripe-shaped mask composed of SiO.sub.2 is formed
on the surface of the p-side contact layer, and the nitride
semiconductor layer(s) are etched by RIE (Reactive Ion Etching)
method using SiCl.sub.4 gas and the stripe-shaped mask. By this
means, a stripe-shaped ridge 16 waveguide region is formed with a
width of 1.5 .mu.m.
[0112] Then, the sides of the ridge are protected by an embedded
film 15 composed of ZrO.sub.2.
[0113] Next, a p-side electrode 17 composed of Ni (10 nm)/Au (100
nm)/Pt (100 nm) is formed on the surface overlying the p-side
contact layer and the embedded film. After that, ohmic annealing is
performed at 600.degree. C. Subsequently, a third protective film
composed of silicon oxide (SiO.sub.2: 0.5 .mu.m thick) is formed by
sputtering on the embedded film, and on the sides of the
semiconductor layer.
[0114] Ni (8 nm)/Pd (200 nm)/Au (800 nm) are formed continuously in
this order on the exposed p-side electrode that is not covered by
the third protective film, to produce a p-side pad electrode
19.
[0115] And then, the surface of the substrate which is opposite to
the side growing the nitride semiconductor layers is polished so as
to have a thickness of 80 .mu.m.
[0116] An n-side electrode 20 composed of V (10 nm)/Pt (200 nm)/Au
(300 nm) is formed on the polished surface of the substrate.
[0117] Next, recessed grooves are formed on the side, on which the
n-side electrode 20 is formed, of the substrate in a wafer state
having the n-side electrode, the p-side electrode and the p-side
pad electrode. These grooves is set to depth of 10 .mu.m, the
length of 50 .mu.m in the direction parallel to the end faces of
the cavity, from the side surface of the nitride semiconductor
layer and the width of 15 .mu.m in the direction perpendicular to
the end faces. The cleaving is performed using the recessed grooves
from the recessed grooves to produce bars in which the cleavage
faces, (1-100) plane, are the cavity end faces.
[0118] On the light exit side of the cavity end faces of the
obtained element, a first protective film 21 is formed, and then a
second protective film 22 is formed on the first protective
film.
[0119] That is, the first protective film 21 (30 nm) which is
composed of AlN is formed at a microwave power of 800 W, RF power
of 800 W, and at a film formation rate of 3 nm/min, a an Ar flow of
30 sccm, and a N.sub.2 flow of 10 sccm with an ECR plasma
sputtering apparatus using an Al target.
[0120] Next, on the first protective film formed on the light exit
face of the cavity, an SiO.sub.2 film is formed as the second
protective film 22 in a thickness of 250 nm with a sputtering
apparatus using an Si target, at a microwave power of 500 W, RF
power of 500 W, and an oxygen flow of 5 sccm.
[0121] On the light reflecting side of the cavity, an
Al.sub.2O.sub.3 film is formed as a protective film 23 in a
thickness of 62 nm, SiO.sub.2/ZrO.sub.2 films are formed as a
protective film 24 in a thickness of 67 nm/44 nm with six cycle
repetition thereon.
[0122] After that, the bar is chipped in the direction
perpendicular to the end faces of the cavity to be formed into a
chip for a semiconductor laser element.
[0123] To check the configuration of the first protective film of
the resulting nitride semiconductor laser element, a cross section
of the nitride semiconductor laser element was observed under a
field emission type of scanning electron microscope (JEM-2010F),
and the bright field TEM image was measured. This can be measured
by directing an electron beam at the first protective film from the
GaN (11-20) plane direction at a camera length of 50 cm.
[0124] FIGS. 4A to 4J show the state of axial orientation at the
various points on the protective film 21 shown in FIG. 3, as
diffraction patterns of the electron beam.
[0125] It can be seen from the diffraction patterns of the electron
beam in FIGS. 4a to 4j that the first protective film in which the
axial orientation is different in the in-plane can be formed on the
cavity end face, depending on the nitride semiconductor layer
composition, as shown in FIG. 3.
[0126] It was also found that as the thickness of the AlN film
increases, there is a change to C axial orientation in the region
where M axial orientation and C axial orientation are both present.
This change to C axial orientation was found to appear at about 5
to 20 nm, depending on the nitride semiconductor layer
composition.
[0127] Thus, how the elements that make up the crystals of the
first protective film are arranged can be visually ascertained from
the resulting electron beam diffraction image.
[0128] Furthermore, when a film near a cavity end face is observed,
diffraction points of the GaN constituting the nitride
semiconductor layer may sometimes be observed. In this case, the
GaN diffraction points can be separated out before analysis.
[0129] The optical output of the resulting semiconductor laser
element before and after continuous high-output oscillation was
measured at a Tc of 80.degree. C., a Po of 320 mW, and an
oscillation wavelength of 405 nm. The results are shown in FIG.
5.
[0130] In FIG. 5, the data indicated by the thin line show the
current-optical output characteristics before high-output
oscillation of the laser element of the present invention, and the
data indicated by the thick line show the current-optical output
characteristics after continuous high-output oscillation of a laser
element.
[0131] According to FIG. 5, it can be seen that the COD level is
kept high, with almost no change, both before and after continuous
high-output oscillation.
[0132] Thus, with the nitride semiconductor laser element of this
Example, no stress is produced in the nitride semiconductor that
makes up the cavity end face, the generation of cracks in the
nitride semiconductor can be prevented, and the end face protective
film adheres well to the cavity end face and does not separate.
This means that a nitride semiconductor laser element with high
performance and output and an increased COD level can be
obtained.
EXAMPLE 2
[0133] A semiconductor laser element is formed by substantially the
same method and constitution as in Example 1, except that, as shown
in FIG. 2B, the protective film of the cavity end face in Example 1
is changed on the rear side to a first protective film 21a composed
of AlN (32 nm) and a second protective film 22a composed of
SiO.sub.2 (250 nm), SiO.sub.2/ZrO.sub.2 films are formed in a
thickness of 67 nm/44 nm with six cycle repetition thereon.
[0134] This semiconductor laser element has the same crystallinity
of the first protective films as in Example 1, and the COD level is
similarly increased.
EXAMPLE 3
[0135] A semiconductor laser element was formed by substantially
the same method and constitution as in Example 1, except that the
protective film of the cavity end face in Example 1 was changed on
the front side to a protective film composed of AlN (20 nm).
[0136] As shown in FIG. 6, a region in which the crystal axial
orientation was different, within the first protective film 31, was
identified in the lamination direction of the nitride semiconductor
layer. The resulting electron beam diffraction images at the
various points in FIG. 6 are substantially the same as those shown
in FIG. 4.
EXAMPLE 4
[0137] A semiconductor laser element was formed by substantially
the same method and constitution as in Example 1, except that the
protective film of the cavity end face in Example 1 was changed on
the front side to a protective film composed of AlN (10 nm).
[0138] As shown in FIG. 7, a region in which the crystal axial
orientation was different, within the first protective film 41, was
identified in the lamination direction of the nitride semiconductor
layer. The resulting electron beam diffraction images at the
various points in FIG. 7 are substantially the same as those shown
in FIG. 4.
EXAMPLE 5
[0139] In Example 5, a nitride semiconductor laser element is
produced in substantially the same manner as in Example 1, except
that when the first protective film 21 composed of AlN was formed
on the cavity end face, the conditions were changed so that the Ar
flow is 50 sccm, the N.sub.2 flow is 10 sccm, the microwave power
is 500 W, the RF power is 500 W, and the film formation rate is 3
nm/min.
[0140] The structure and characteristics of the first protective
film of the nitride semiconductor laser element thus obtained are
substantially the same as those in Example 1.
EXAMPLE 6
[0141] In Example 6, a nitride semiconductor laser element is
produced in substantially the same manner as in Example 1, except
that when the first protective film 21 composed of AlN was formed
on the cavity end face, the conditions were changed so that the Ar
flow is 30 sccm, the N.sub.2 flow is 6 sccm, the microwave power is
500 W, the RF power is 500 W, and the film formation rate is 3
nm/min.
[0142] The structure and characteristics of the first protective
film of the nitride semiconductor laser element thus obtained are
substantially the same as those in Example 1.
EXAMPLE 7
[0143] In Example 7, a nitride semiconductor laser element is
produced in substantially the same manner as in Example 1, except
that when the first protective film 21 composed of AlN was formed
on the cavity end face, the conditions were changed so that firstly
the Ar flow is 30 sccm, the N.sub.2 flow is 10 sccm, the microwave
power is 800 W, the RF power is 800 W, and the film formation rate
is 3 nm/min, and then, the Ar flow is 30 sccm, the N.sub.2 flow is
10 sccm, the microwave power is 500 W, the RF power is 500 W, and
the film formation rate is 2 nm/min.
[0144] The structure and characteristics of the first protective
film of the nitride semiconductor laser element thus obtained are
substantially the same as those in Example 1.
EXAMPLE 8
[0145] In Example 8, a nitride semiconductor laser element is
produced in substantially the same manner as in Example 1, except
that when the first protective film 21 composed of AlN was formed
on the cavity end face, the conditions were changed so that firstly
the Ar flow is 50 sccm, the N.sub.2 flow is 10 sccm, the microwave
power is 500 W, the RF power is 500 W, and the film formation rate
is 3 nm/min, and then, the Ar flow is 30 sccm, the N.sub.2 flow is
10 sccm, the microwave power is 500 W, the RF power is 500 W, and
the film formation rate is 2 nm/min.
[0146] The structure and characteristics of the first protective
film of the nitride semiconductor laser element thus obtained are
substantially the same as those in Example 1.
EXAMPLE 9
[0147] In Example 9, a nitride semiconductor laser element is
produced in substantially the same manner as in Example 1, except
that when the first protective film 21 composed of AlN was formed
on the cavity end face, the conditions were changed so that firstly
the Ar flow is 30 sccm, the N.sub.2 flow is 6 sccm, the microwave
power is 500 W, the RF power is 500 W, and the film formation rate
is 3 nm/min, and then, the Ar flow is 30 sccm, the N.sub.2 flow is
10 sccm, the microwave power is 500 W, the RF power is 500 W, and
the film formation rate is 2 nm/min.
[0148] The structure and characteristics of the first protective
film of the nitride semiconductor laser element thus obtained are
substantially the same as those in Example 1.
EXAMPLE 10
[0149] In Example 10, a nitride semiconductor laser element is
produced in substantially the same manner as in Example 1, except
that when the first protective film 21 composed of AlN was formed
on the cavity end face, the conditions were changed so that firstly
the Ar flow is 30 sccm, the N.sub.2 flow is 10 sccm, the microwave
power is 500 W, the RF power is 500 W, and the film formation rate
is 2 nm/min, and then, the wafer keep a distance of 20 mm from the
target, the Ar flow is 30 sccm, the N.sub.2 flow is 10 sccm, the
microwave power is 500 W, the RF power is 500 W, and the film
formation rate is 1.7 nm/min.
[0150] The structure and characteristics of the first protective
film of the nitride semiconductor laser element thus obtained are
substantially the same as those in Example 1.
EXAMPLE 11
[0151] In Example 11, a semiconductor laser element was formed by
substantially the same method and constitution as in Example 1,
except that the first protective film 21 of the cavity end face in
Example 1 was changed to a thickness of 10 nm.
[0152] The axial orientation of the first protective film of the
resulting nitride semiconductor laser element changed hardly at all
in the thickness direction of the protective film, regardless of
which nitride semiconductor layer it was over, and the same changes
as in Example 1 were noted, only in the in-plane direction of the
first protective film.
EXAMPLE 12
[0153] A semiconductor laser element is formed by substantially the
same method and constitution as in Example 1, except the
followings; the protective film of the cavity end face on the front
side in Example 1 is changed to a first protective film 21 composed
of AlN (30 nm), the first protective film is formed under the
conditions that the Ar flow is 50 sccm, the N.sub.2 flow is 5 sccm,
the microwave power is 800 W, the RF power is 800 W, and the film
formation rate is 7 nm/min.
[0154] The nitride semiconductor laser element obtained in Example
12 was observed with a field emission type of transmission electron
microscope in the same manner as in Example 1. FIG. 8 is a
schematic view of this.
[0155] It can be seen in FIG. 8 that the axial orientation in the
in-plane is different with respect to the cavity end face depending
on the composition of the nitride semiconductor layer, and that the
first protective film whose crystal orientation is different from
that of the cavity plane of the nitride semiconductor layer can be
formed over the majority of the nitride semiconductor layer
including the InGaN layer.
[0156] Also, it was found that as the thickness of the first
protective film (AlN) increases, there is a change to C axial
orientation in the region where M axial orientation and C axial
orientation are both present. This change to C axial orientation
was seen to appear at about 5 to 20 nm depending on the composition
of the nitride semiconductor layer.
[0157] The nitride laser element thus obtained was measured for
optical output in the same manner as in Example 1, whereupon
substantially the same results were obtained as in Example 1.
[0158] Thus, with the nitride semiconductor laser element of this
Example 12, it is possible to obtain the end face protective film
with which there is no stress produced in the nitride semiconductor
that makes up the cavity end face, cracking in the nitride
semiconductor is prevented, adhesion with the cavity end face is
good, and separation is prevented. It was confirmed that this
allows a nitride semiconductor laser element to be obtained with an
increased COD level, higher performance, and higher output.
EXAMPLE 13
[0159] A semiconductor laser element is formed by substantially the
same method and constitution as in Example 1, except the
followings; the active layer is formed of a multi quantum well
structure (MQW)) with a total thickness of 72 nm through that a
barrier layer 13b composed of In.sub.0.02Ga.sub.0.98N doped with Si
(14 nm thick), a well layer 13a composed of undoped
In.sub.0.07Ga.sub.0.93N (8 nm thick) are laminated on the barrier
layer 13b and this process is repeated 2 times, finally the barrier
layer 13b composed of undoped In.sub.0.02Ga.sub.0.98N (28 nm thick)
is formed on the layers, and the protective film of the cavity end
face on the front side in Example 1 is changed to a first
protective film 21 composed of AlN (30 nm), the first protective
film is formed under the conditions that the Ar flow is 50 sccm,
the N.sub.2 flow is 5 sccm, the microwave power is 800 W, the RF
power is 800 W, and the film formation rate is 7 nm/min.
[0160] The structure and characteristics of the first protective
film of the nitride semiconductor laser element thus obtained are
substantially the same as those in Example 1.
EXAMPLE 14
[0161] A semiconductor laser element of this Example 14 can be
manufactured by the following method according to the method of
Example 1.
[0162] First, an n-layer composed of Al.sub.0.02Ga.sub.0.98N doped
with Si at 4.times.10.sup.18/cm.sup.3 (1 .mu.m thick) is grown on
the GaN substrate at a growth temperature of 1100.degree. C. using
TMA, TMG and ammonia as the raw material gas with a silane gas for
an impurity gas. Nest, In.sub.0.05Ga.sub.0.95N doped with Si (0.15
.mu.m thick) is grown at a growth temperature of 930.degree. C.,
and an n-side clad layer composed of Al.sub.0.06Ga.sub.0.94N doped
with Si at 4.times.10.sup.18/cm.sup.3 (2 .mu.m thick) is grown.
This n-side clad layer may be composed of a super lattice
structure.
[0163] Next, an n-side wave guide layer composed of undoped GaN
(0.3 .mu.m thick) is grown at a growth temperature of 1000.degree.
C. This wave guide layer 12a may be doped partially or wholly with
n-type impurities.
[0164] The temperature is set to 900.degree. C., a first barrier
layer composed of In.sub.0.02Ga.sub.0.98N doped with Si (70 nm
thick), an undoped GaN (1 nm thick) on the first barrier layer, and
at the temperature to 850.degree. C., a well layer composed of
undoped In.sub.0.13Ga.sub.0.87N (3 nm thick) are laminated. The
temperature is set to 900.degree. C., undoped GaN (14 nm thick),
and at the temperature to 850.degree. C., a well layer composed of
undoped In.sub.0.13Ga.sub.0.87GaN (3 nm thick) are laminated.
Finally the barrier layer composed of undoped
In.sub.0.02Ga.sub.0.98N (70 nm thick) is laminated.
[0165] A p-side cap layer is formed the same method as in Example
1.
[0166] Next, Cp.sub.2Mg gas and TMA are stopped, and p-side wave
guide layer composed of undoped GaN (0.3 .mu.m thick) is grown at a
growth temperature of 1000.degree. C. This p-side wave guide layer
may be doped partially or wholly with p-type impurities.
[0167] A p-side clad layer and a p-side contact layer (15 nm thick)
are formed the same method as in Example 1.
[0168] The resulting wafer on which the nitride semiconductor has
been grown is taken out of the reaction vessel, a stripe-shaped
structure of the nitride semiconductor which will be main body of
the cavity in the laser element is formed the same method as in
Example 1.
[0169] Next, a stripe-shaped mask composed of SiO.sub.2 is formed
on the surface of the p-side contact layer, and the nitride
semiconductor layer(s) are etched by RIE method using SiCl.sub.4
gas and the stripe-shaped mask. By this means, a stripe-shaped
ridge waveguide region is formed with a width of 2.0 .mu.m.
[0170] Then, the sides of the ridge are protected by an embedded
film 15 composed of ZrO.sub.2 (200 nm thick).
[0171] Next, a p-side electrode 17 and a third protective film is
formed the same method as in Example 1. After that, ohmic annealing
is performed.
[0172] A p-side pad electrode is formed and the surface of the
substrate is polished the same method as in Example 1.
[0173] On the light exit side of the cavity end faces, a first
protective film 21 is formed, and then a second protective film 22
is formed on the first protective film.
[0174] That is, the first protective film 21 (10 nm) which is
composed of AlN is formed at a microwave power of 800 W, RF power
of 800 W, and at a film formation rate of 3 nm/min, an Ar flow of
30 sccm, and a N.sub.2 flow of 10 sccm with an ECR plasma
sputtering apparatus.
[0175] Next, on the first protective film formed on the light exit
face of the cavity, a SiO.sub.2 film is formed as the second
protective film 22 in a thickness of 295 nm with a sputtering
apparatus using an Si target, at a microwave power of 500 W, RF
power of 500 W, and an oxygen flow of 5 sccm.
[0176] On the light reflecting side of the cavity, an ZrO.sub.2
film is formed in a thickness of 49 nm, and SiO.sub.2/ZrO.sub.2
films are formed in a thickness of 75 nm/49 nm with six cycle
repetition thereon.
[0177] After that, the bar is chipped in the direction
perpendicular to the end faces of the cavity to be formed into a
chip for a semiconductor laser element.
[0178] The structure and characteristics of the first protective
film of the nitride semiconductor laser element thus obtained are
substantially the same as those in Example 1.
EXAMPLE 15
[0179] A semiconductor laser element of this Example is formed by
substantially the same method and constitution as in Example 1,
except that, end face protective film in Example 1 is changed to a
film composed of AlN (32 nm), SiO.sub.2 (260 nm) and ZrO.sub.2 (45
nm) in this order from the end face side of the cavity.
[0180] The structure and characteristics of the first protective
film of the nitride semiconductor laser element thus obtained are
substantially the same as those in Example 1.
[0181] The present invention can be applied to a wide range of
nitride semiconductor elements with which the protective film needs
to adhere well to the semiconductor layer, such as in use as light
emitting elements (e.g., LD, LED, super luminescence diode, etc.),
solar cells, light-receptive elements (e.g., light sensor, etc.),
electric devices (e.g., transistor, power device, etc.) and the
like. In particular, it is useful as nitride semiconductor elements
in optical disk applications, optical communications systems,
printers, optical exposure applications, and various devices for
measurement, excitation light source for bio-specific applications
and the like.
[0182] While only selected embodiments have been chosen to
illustrate the present invention, it will be apparent to those
skilled in the art from this disclosure that various changes and
modifications can be made herein without departing from the scope
of the invention as defined in the appended claims. Furthermore,
the foregoing descriptions of the embodiments according to the
present invention are provided for illustration only, and not for
the purpose of limiting the invention as defined by the appended
claims and their equivalents. Thus, the scope of the invention is
not limited to the disclosed embodiments.
* * * * *